Vehicles equipped with diesel engines may include exhaust systems that have diesel particulate filters for removing particulate matter from the exhaust stream. With use of the diesel particulate filters, soot or other carbon-based particulate matter may accumulate on the filters. As particulate matter accumulates on the diesel particulate filters, the restriction of the filters increases, causing the buildup of undesirable back pressure in the exhaust systems. High back pressures decrease engine efficiency and reduce engine performance. Therefore, to prevent diesel particulate filters from becoming excessively loaded, diesel particulate filters should be regularly regenerated by burning off (i.e., oxidizing) the particulates that accumulate on the filters. Under most diesel engine operating conditions, however, the engine exhaust temperature is too low to cause the diesel particulate filter to completely self-regenerate. Thus, it is necessary to provide a means for initiating regeneration of the diesel particulate filter.
There are a number of methods for regenerating diesel particulate filters known to those skilled in the art. One known method is to operate the engine fuel injection apparatus so as to inject a quantity of fuel late in the combustion stroke of the engine piston, causing the fuel to burn and raise the exhaust temperature sufficiently to initiate regeneration without substantially increasing the engine output torque. Alternatively, a diesel particulate filter may be heated by an electrical heating element to a temperature sufficient to initiate regeneration. Although these systems are generally effective for initiating regeneration of a diesel particulate filter, each has certain drawbacks in application.
Another method for regenerating a diesel particulate filter involves positioning a fuel injector and an oxidation catalyst upstream of a diesel particulate filter. To initiate regeneration, the fuel injector injects hydrocarbon fuel into the exhaust stream, which is oxidized in the oxidation catalyst to raise the temperature of the exhaust stream sufficiently to initiate regeneration of the diesel particulate filter. An example of such a system is disclosed in U.S. patent application Ser. No. 11/016,345, filed Dec. 16, 2004, which is herein incorporated by reference in its entirety.
Diesel exhaust contains nitrogen oxides (NOx), which consist primarily of nitric oxide (NO) and nitrogen dioxide (NO2). Typically, the NO2 in the exhaust stream is a relatively small percentage of total NOx, such as in the range of 5 to 20 percent but usually in the range of 5 to 10 percent. Although nitrogen oxides have been a regulated constituent of diesel exhaust for some time, recent developments have suggested that emissions of NO2 should be regulated separately from overall NOx emissions for environmental and health reasons. Therefore, it is desired that a diesel exhaust treatment system does not cause excessive increases in the amount of NO2 within the exhaust stream. One regulation proposed in California requires that the ratio of NO2 to NOx in the exhaust gas downstream from an exhaust treatment system be no more than 20 percent greater than the ratio of NO2 to NOx in the exhaust gas upstream from the exhaust treatment system. In other words, if the engine-out NOx mass flow rate is (NOx)eng, the engine-out NO2 mass flow rate is (NO2)eng, and the exhaust-treatment-system-out NO2 mass flow rate is (NO2)sys, then the ratio
                    (                  NO          ⁢                                          ⁢          2                )            ⁢      sys        -                  (                  NO          ⁢                                          ⁢          2                )            ⁢      eng                  (              NO        ⁢        x            )        ⁢    eng  must be less than 0.20.
An exhaust treatment system that includes a diesel oxidation catalyst will typically oxidize some of the NO present in the exhaust to form NO2. Moreover, because the exhaust typically flows through the oxidation catalyst at all times, and not only when the diesel particulate filter is being regenerated, the oxidation catalyst will typically cause a significant overall increase in the amount of NO2 emissions. Although total NOx emissions will generally remain the same, this increase in NO2 may be problematic under proposed diesel exhaust emissions regulations. Therefore, it is desired to create a diesel exhaust treatment system that provides for the regeneration of a diesel particulate filter without excessively increasing NO2 emissions.